[Show abstract][Hide abstract] ABSTRACT: We report a thermally activated metastability in a GaAs double quantum dot
exhibiting real-time charge switching in diamond shaped regions of the charge
stability diagram. Accidental charge traps and sensor back action are excluded
as the origin of the switching. We present an extension of the canonical double
dot theory based on an intrinsic, thermal electron exchange process through the
reservoirs, giving excellent agreement with the experiment. The electron spin
is randomized by the exchange process, thus facilitating fast, gate-controlled
spin initialization. At the same time, this process sets an intrinsic upper
limit to the spin relaxation time.
[Show abstract][Hide abstract] ABSTRACT: We present a method for determining correlations in a gas of indirect
excitons in a semiconductor quantum well structure. The method involves
subjecting the excitons to a periodic electrostatic potential that causes
modulations of the exciton density and photoluminescence (PL). Experimentally
measured amplitudes of energy and intensity modulations of exciton PL serve as
an input to a theoretical estimate of the exciton correlation parameter and
temperature. We also present a proof-of-principle demonstration of the method
for determining the correlation parameter and discuss how its accuracy can be
Physical Review B 08/2015; 92(11). DOI:10.1103/PhysRevB.92.115311 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Indirect excitons in coupled quantum wells are long-living quasiparticles, explored in the studies of collective quantum states. We demonstrate that, despite the extremely low oscillator strength, their spin and population dynamics can by addressed by time-resolved pump-probe spectroscopy. Our experiments make it possible to unravel and compare spin dynamics of direct excitons, indirect excitons, and residual free electrons in coupled quantum wells. Measured spin relaxation time of indirect excitons exceeds not only one of direct excitons but also one of free electrons by two orders of magnitude.
Physical Review B 03/2015; 91(12):125437. DOI:10.1103/PhysRevB.91.125437 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present experimental proof of principle for two-dimensional electrostatic traps for indirect excitons. A confining trap potential for indirect excitons is created by a snowflake-shaped electrode pattern. We demonstrate collection of indirect excitons from all directions to the trap center and control of the trap potential by voltage.
[Show abstract][Hide abstract] ABSTRACT: At low temperatures, indirect excitons formed at the in-plane electron-hole
interface in a coupled quantum well structure undergo a spontaneous transition
into a spatially modulated state. We report on the control of the instability
wavelength, measurement of the dynamics of the exciton emission pattern, and
observation of the fluctuation and commensurability effect of the exciton
density wave. We found that fluctuations are strongly suppressed when the
instability wavelength is commensurate with defect separation along the exciton
density wave. The commensurability effect is also found in numerical
simulations within the model describing the exciton density wave in terms of an
instability due to stimulated processes.
Physical Review B 02/2015; 91(24). DOI:10.1103/PhysRevB.91.245302 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We demonstrate fast universal electrical spin manipulation with inhomogeneous
magnetic fields. With fast Rabi frequency up to 127 MHz, we leave the
conventional regime of strong nuclear-spin influence and observe a spin-flip
fidelity > 96%, a distinct chevron Rabi pattern in the spectral-time domain,
and spin resonance linewidth limited by the Rabi frequency, not by the
dephasing rate. In addition, we establish fast z-rotations up to 54 MHz by
directly controlling the spin phase. Our findings will significantly facilitate
tomography and error correction with electron spins in quantum dots.
[Show abstract][Hide abstract] ABSTRACT: Collective vibrations of proteins, rotations of small molecules, excitations
of high-temperature superconductors, and electronic transitions in
semiconductor nanostructures occur with characteristic frequencies between 1
and 10 THz. Applications to medicine, communications, security and other fields
are emerging. However, mapping the coldest parts of the universe has been the
largest driver for developing THz detectors. The result is a family of
exquisitely-sensitive detectors requiring sub-4K temperatures. For earthbound
THz science and technology, sensitivity remains important but many applications
require high speed and operating temperatures. Room-temperature Schottky diodes
enable some of these applications. Here we demonstrate a new type of detector
in which THz radiation excites a collective oscillation of ~25,000 electrons
between two gates in a microscopic four terminal transistor. The energy
dissipates into other modes of the electron gas, warming it and changing the
source-drain resistance. The detector shows amplifier-limited rise times near 1
ns and has detected THz laser radiation at temperatures up to 120K. The
frequency of the collective oscillation tunes with small gate voltages. The
first-generation tunable antenna-coupled intersubband Terahertz (TACIT)
detectors tune between 1.5 and 2 THz with voltages <2V.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate experimental proof of principle for a stirring potential for
indirect excitons. The azimuthal wavelength of this stirring potential is set
by the electrode periodicity, the amplitude is controlled by the applied AC
voltage, and the angular velocity is controlled by the AC frequency.
[Show abstract][Hide abstract] ABSTRACT: Solid-state qubits have recently advanced to the level that enables them,
in-principle, to be scaled-up into fault-tolerant quantum computers. As these
physical qubits continue to advance, meeting the challenge of realising a
quantum machine will also require the engineering of new classical hardware and
control architectures with complexity far beyond the systems used in today's
few-qubit experiments. Here, we report a micro-architecture for controlling and
reading out qubits during the execution of a quantum algorithm such as an error
correcting code. We demonstrate the basic principles of this architecture in a
configuration that distributes components of the control system across
different temperature stages of a dilution refrigerator, as determined by the
available cooling power. The combined setup includes a cryogenic
field-programmable gate array (FPGA) controlling a switching matrix at 20
millikelvin which, in turn, manipulates a semiconductor qubit.
[Show abstract][Hide abstract] ABSTRACT: We present new high-resolution measurements of transient time-domain photoconductivity in ErAs:InGaAs superlattice nanocomposites intended for THz photoconductive switches and photomixers using a pure optical pump-probe method. We developed a model, using separate photocarrier trapping, recombination, and thermal reactivation processes, which very accurately fits the measurements. The measured material structures all exhibit a slow secondary decay process, which is attributed to thermal reactivation of the trapped carriers, either into the conduction band, or into high-energy defect states. We examined the influence of superlattice structure, dopants, DC bias, and temperature. Analysis shows that all of the THz energy produced by the photocarrier trapping and decay processes are at frequencies less than 1 THz, while the reactivation process only serves to create a large portion of the bias power dissipated. Energy higher than 1 THz must be created by a fast generation process or band-filling saturation. This allows pulsed THz generation even from a long-lifetime material. Pure optical pump-probe measurements are necessary to expose slow material processes, and eliminate the influence of electrical terminals and THz antennas. These measurements and modeling of THz photoconductive devices are necessary in order to optimize the output spectrum and power.
[Show abstract][Hide abstract] ABSTRACT: We report on self-assembled ErSb nanowires in a GaSb matrix that show a strong polarization-sensitive THz response. The nanowires behave like a polarizer. Their orientation and shape can be engineered by the growth conditions.
[Show abstract][Hide abstract] ABSTRACT: We report the observation of spin currents and spin polarization textures in opti- cally generated indirect excitons. The textures are observed in linear and circular polarizations and are controlled by magnetic fields.
[Show abstract][Hide abstract] ABSTRACT: We experimentally demonstrate an order of magnitude higher radiated power from a 1550 nm photomixer with plasmonic contact electrodes in comparison with an analogous photomixer without plasmonic contact electrodes in the 0.25-2.5 THz frequency range.
[Show abstract][Hide abstract] ABSTRACT: While the growth of III-As and III-P semiconductors is well-established, and their transport properties well-understood, the performance of high-frequency and VLSI electron devices can still be substantially improved. Here we review design principles, experimental efforts, and intermediate results, in the development of nm and THz electron devices, including nm InAs/InGaAs planar MOSFETs and finFETs for VLSI, InGaAs/InP DHBTs for 0.1-1 THz wireless communications and imaging, and ~5nm InAs/InGaAs Schottky diodes for mid-IR mixing.
2014 72nd Annual Device Research Conference (DRC); 06/2014
[Show abstract][Hide abstract] ABSTRACT: The compound semiconductor gallium-arsenide (GaAs) provides an ultra-clean platform for storing and manipulating quantum information, encoded in the charge or spin states of electrons confined in nanostructures. The absence of inversion symmetry in the zinc-blende crystal structure of GaAs however, results in a strong piezoelectric interaction between lattice acoustic phonons and qubit states with an electric dipole, a potential source of decoherence during charge-sensitive operations. Here we report phonon generation in a GaAs double quantum dot, configured as a single- or two-electron charge qubit, and driven by the application of microwaves via surface gates. In a process that is a microwave analogue of the Raman effect, phonon emission produces population inversion of the two-level system and leads to rapid decoherence of the qubit when the microwave energy exceeds the level splitting. Comparing data with a theoretical model suggests that phonon emission is a sensitive function of the device geometry.
[Show abstract][Hide abstract] ABSTRACT: Multielectron spin qubits are demonstrated, and performance examined by comparing coherent exchange oscillations in coupled single-electron and multielectron quantum dots, measured in the same device. Fast (>1 GHz) exchange oscillations with a quality factor Q∼15 are found for the multielectron case, compared to Q∼2 for the single-electron case, the latter consistent with experiments in the literature. A model of dephasing that includes voltage and hyperfine noise is developed that is in good agreement with both single- and multielectron data, though in both cases additional exchange-independent dephasing is needed to obtain quantitative agreement across a broad parameter range.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate a low loss, chip-level frequency multiplexing scheme for
readout of scaled-up spin qubit devices. By integrating separate bias tees and
resonator circuits on-chip for each readout channel, we realize dispersive
gate-sensing in combination with charge detection based on two rf quantum point
contacts (rf-QPCs). We apply this approach to perform multiplexed readout of a
double quantum dot in the few-electron regime, and further demonstrate
operation of a 10-channel multiplexing device. Limitations for scaling spin
qubit readout to large numbers of multiplexed channels is discussed.